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Deutsche Forschungsgemeinschaft

Abstract

The oxidation state of magmas is a parameter of prime importance in magmatic processes. Despite various existing techniques its reconstruction remains a challenging task, particularly in the case of intrusive rocks. This is because in such rocks the mineral phases that are sensitive to oxygen fugacity were either destroyed or reset at subsolidus conditions, such that accurate estimation of magmatic fO2 is not possible. Thus, the aim of this study is to develop and apply new proxies for magmatic oxidation state (i.e. oxybarometers) that can be used also in rocks that were affected by postmagmatic alteration processes. In this thesis two such independent methods are presented that are based (i) on the partitioning of vanadium, as well as (ii) the exchange of iron and titanium between magnetite and silicate melt. The thesis includes their experimental calibration as well as their first application to natural rocks.
In order to calibrate the new oxybarometers a series of experiments were carried out at varying oxygen fugacities (0.7-4.0 log units above the fayalite-magnetite-quartz buffer), temperatures (800-1000 °C), melt alumina saturation indices (ASI=0.74-1.14), magnetite composition (0.2-14 wt% TiO2) and pressure (1-5 kbar; at H2O saturation). The experiments were performed by equilibrating small (≤20 µm), V-free magnetite grains in V-doped silicate melts (~100 ppm V). Both phases were analyzed by LA-ICP-MS and partition coefficients of vanadium as well as exchange coefficients of Fe and Ti were obtained between magnetite and silicate melt. Attainment of equilibrium was demonstrated by reverse experiments.
The experimental results suggest that DVmgt/melt depends strongly on oxygen fugacity, to a smaller (but still considerable) degree on melt alumina saturation index and temperature. In contrast, magnetite composition and melt water content seem to have negligible effects on vanadium partitioning. Thus, DVmgt/melt can be expressed as a function of oxygen fugacity, temperature and melt composition in the form of a simple equation. This equation reproduces all our experimental DVmgt/melt values within 0.3 log units, and 89% of them within 0.15 log units.
The experimentally calibrated vanadium partitioning oxybarometer was applied to a series of natural rhyolites and dacites. The investigated samples included vitrophyres and holocrystalline rocks in which part of the mineral- and melt assemblage was preserved only as inclusions within phenocrysts. An independent fO2 constraint for vanadium magnetite–melt oxybarometry was obtained via Fe–Ti-oxide oxybarometry, whereas temperature was constrained by zircon saturation thermometry, two-feldspar thermometry and Fe–Ti-oxide thermometry. All analyses were conducted by laser-ablation ICP-MS. In most of the samples the fO2 values determined via vanadium magnetite–melt oxybarometry agree within 0.5 log units with the oxygen fugacity calculated from Fe-Ti-oxide pairs, except for a few cases where the larger discrepancy can be explained by magma mixing processes. The fO2 value obtained by vanadium partitioning depends significantly on the applied thermometer. Temperatures based on zircon saturation thermometry and two-feldspar thermometry usually agreed within the limits of uncertainty, whereas temperatures obtained via Fe–Ti-oxide thermometry commonly deviated by ≥50 C due to large uncertainties associated with the Fe–Ti-oxide model at T-fO2 conditions typical of most silicic magmas. Therefore, the former two methods are recommended to constrain temperature for vanadium partitioning oxybarometry. The main advantages of this new oxybarometer over classical magnetite–ilmenite oxybarometry are (1) that it can be applied to rocks that do not contain ilmenite, and (2) that it is easier to apply to slowly-cooled rocks such as granites by measuring magnetite-melt pairs in form of inclusions.
Our experimental data was extended by experimental magnetite- and ilmenite-bearing samples from the literature, covering a wide range of oxygen fugacities, temperatures, pressures and silicate melts ranging from basaltic to rhyolitic in composition. Using this extended dataset a further oxybarometer could be calibrated that is based on the partitioning of Fe and Ti between magnetite and melt (i.e. the Fe–Ti exchange coefficient) and is therefore named FeTiMM. In the case of FeTiMM oxygen fugacity was shown to depend solely on the Fe–Ti exchange coefficient and melt composition. The fitting equation based on these two variables yielded fO2 values that mostly agree within 0.5 log units with the fO2, independently constrained by Fe–Ti-oxide oxybarometry, the performance of FeTiMM being similarly good on felsic, mafic and intermediate melts. A first test of the method on natural samples of dacitic to rhyolitic compositions yielded consistent results with Fe–Ti oxide oxybarometry and vanadium partitioning oxybarometry alike. FeTiMM thus opens the door for numerous new applications in various disciplines of Earth Sciences, including the fields of volcanology, igneous petrology, experimental geochemistry, and ore geology. The main advantages of FeTiMM are (1) that it is applicable to both ilmenite-free and ilmenite-bearing samples (2) that it can be applied even to slowly-cooled intrusive rocks such as granites (3) that it is temperature-independent and (4) that it is calibrated to and is therefore applicable to a broad range of melt compositions, spanning the entire range from basalts to rhyolites.